Method of treating the surface of a filament

ABSTRACT

THE METHOD OF INCREASING THE INTERLAMINAR SHEAR STRENGTH CAPABILITIES OF A MATERIAL TO BE BONDED TO ANOTHER MATERIAL BY GROWING INTEGRALLY-ATTACHED MONOCRYSTALLINE WHISKERS OUT FROM THE SURFACE OF THE FIRST MATERIAL BOTH TO ROUGHEN ITS SURFACE FOR BONDING TO A DEPOSITED MATRIX MATERIAL, OR TO A PLASTIC, AND TO CHANGE ITS SURFACE CHEMISTRY TO IMPROVE BONDING, FOR INSTANCE TO A RESIN. THE INVENTION INCLUDES THE REINFORCING A FIBEROUS STRUCTURES BY THE INTERTWINING OF WHISKERS GROWN ACROSS VOIDS THEREBETWEEN, AND FURTHER INCLUDES VARIOUS WHISKER-GROWN STRUCTURES PER SE.

y 1971 .LVMILEWSK! ETAL 3,580,731

METHOD OF TREATING THE SURFACE OF A FILAMENT Filed Sept. 26, 1967 2Sheets-Sheet 1 f {I lA/l/ INVENTORY JOHN v. MILEWSKI JMES J.S E

ATTORNEYS May 25, 1971 J. v. MILEWSKI ETAL 3,580,731

METHOD OF TREATING THE SURFACE OF A FILAMENT Filed Sept. 26, 196? 2Shees-Sheet 2 uvmvrom JOHN V. MILEWSKI JAMES J. S BY 1 m ATTORNEYSUnited States Patent 3,580,731 METHOD OF TREATING THE SURFACE OF AFILAMENT John V. Milewski, Saddle Brook, and James J. Shyne,

Caldwell, NJ., assignors to General Technologies Corporation Filed Sept.26, 1967, Ser. No. 670,628 Int. Cl. C01b 31/30,- C23c 11/08 U.S. Cl.117-66 14 Claims ABSTRACT OF THE DISCLOSURE This invention relates toimprovements in the structure of laminate members such as fibrils,yarns, bundles of fibers, mats, woven reinforcing fabrics, and rigidmembers having surfaces intended to be bonded to other matenals, theseimprovements being for the purpose of increasing the strength of thebond by growing whiskers upon external surfaces so that they bond moresecurely to the various matrix filler materials to form high-strengthcomposites. The invention also relates to novel processes for making theimproved surface structures by growing monocrystalline whiskers normalto the surfaces.

The superior modulus of a polycrystalline fiber is contributed to bysurface perfection which results in high surface tension, but at thesame time, a very smooth surface. When these fibers are laminated thebond is the weakest feature of the combination, and tests show that sucha composite readily fails by delamination, generally referred to assurface interlaminar failure.

A salient example of the above problem is found in polycrystallinecarbon fibers whose Youngs modulus is very high, around 25 to 50 millionas compared with Fiberglas whose modulus is about million, but thecarbon surface bond capability with matrix materials is particularlypoor. Carbon is not only a lighter material, weighing only abouttwo-thirds as much as glass fiber, but it is more inert chemically andhas a high decomposition point of about 6000 F. It is easily made bydecomposing rayon thread or fabric in a reducing atmosphere, or it canbe brought from several sources, for instance under the trade nameThornel from Union Carbide Company. However, information generated atthe Naval Ordnance Laboratory, and published in several articlesappearing in Nature Magazine in 1966 and 1967 showed in variouscomposites an inverse correlation between interlaminar shear-strengthand the Youngs modulus of the carbon reinforcements, which relationshipsuggested a dim future for carbon-fiber composites unless a way weredeveloped to vastly improve their bond to resins.

In various experimental programs, others have tried to improve thesurface bond of carbon fibers variously by chemical surface treatment,etching, dip and bake, coating, roughening, etc., but these effortsresulted only in modest degrees of improvement, perhaps up to about 30%.The present invention has provided a solution of the problem byobtaining an improvement in the surface bond in the neighborhood of 500%or more, while proice viding a number of additional fringe benefitswhich will be set forth below.

It is the principal object of the invention to treat surfaces byWhiskerizing them to produce an improved capability of the surface tosecurely bond to another material, such as a matrix material.

It is another major object of the invention to prepare the surfaces ofindividual fibers for secure bonding without appreciably stiffening thefibers, as is the case when they were coated according to prior artefforts.

Another important object of the invention is to grow whiskers on theinndividual fibrils within a fiber bundle, thread, or woven clothwherein the whiskers are integrally attached thereto and extendoutwardly generally normal to the surfaces, thus providingreinforcements in three dimensions, and in such lengths that thewhiskers grown from adjacent fibrils overlap and intert-wine to provideenormous increases in interlaminar sheer strength. Typically, a bundleof carbon fibers forming a thread includes 720 or 1440 individualfibrils, each of which is about 10 microns in diameter and is ofpolycrystalline structure. The whiskers grown on these fibrils are,however, only about .01 to 1 micron in diameter in the case of siliconcarbide whiskers, and these whiskers comprise monocrysalline growthswhose modulus is much higher because being monocrystalline, they arefree of grain boundaries, surface defects, dislocations, and impurities.They enjoy a strength which is obtainable only because of their highdegree of perfection. As a result, when these whiskers are stressed, thepull is against atomic cohesion forces, rather than against the multipleimperfections which characterize polycrystalline structures. The formerapproach theoretical strength while the latter always fall short of itby many orders of magnitude.

Still a further object of this invention is to provide fiberouslaminating structures wherein the whiskers have been grown between theindividual fibrils and with such length and population density as tocross, intertwine, and occupy all of the interstitial voids locatedbetween fibrils, or between adjacent bundles of fibers, and/or betweenthreads in a woven or matted material, whereby such whisker treatedreinforcing products have a considerable degree of cohesion even beforethey are impregnated with matrix filler materials, i.e. resins.

Still a further object of this invention is to provide whisker treatedfilamentary products wherein whisker lengths are controlled to suitvarious different needs. For example in a bundle of fibrils as set forthabove, whiskers can be grown say to 1'0 bundle diameters making themuseful for some types of matrix composite service, i.e., whereinterstitial voids are large. Alternatively, the whiskers can be grownto much shorter lengths where more fibril surface roughening is desiredbut the whiskers are not needed as criss-crossing members in the resinoccupied interstices. The presence of a dense growth of long whiskers isan advantage in high temperature applications, such as reinforcement ofmetal matrix rocket motor parts, because the pure monocrystalline natureof the whiskers makes them especially resistant to ablation.Nevertheless, there is such a thing as having the whisker growth in theinterstices too dense, thus making it impossible to obtain completepenetration. The ratio of whisker length to diameter is important sinceit affects their tendency to interweave, tangle, bend or repel eachother like little springs, i.e., packing factor. The whisker materialitself is also important since it affects the general characfibrilswhich have high Youngs modulus can be success 3 fully used and even spuninto yarn for applications where formerly only long fibers or continuousfilaments have been used.

Another major object of this invention is to provide techniques andprocesses for growing whiskers of controlled lengths and of selectedmaterials upon fibrils, bundles, threads or weaves of various othersimilar or dissimilar materials using gas-transfer-mechanism techniquesto grow whiskers of non-vaporizing materials in either batch orcontinuous runs.

The growing of whiskers is being done currently to obtain the whiskersthemselves, the whiskers being grown as a crop upon slabs of sheetmaterial and then broken off and collected for use as reinforcements invarious matrix materials. These unattached whiskers have been used tostrengthen high performance structures, such as turbine blades, andefforts are being made to spin the unattached whiskers into yarns. Theyare also being made into mats, papers, and wool-like forms. However, theimmediate object of this invention is to grow the whiskers directly uponsurfaces to which the Whiskers remain integrally attached duringimpregnation within a suitable matrix material, or bonding of theWhiskerized surface to another material. A synergistic effect results,which provides a novel surface whose own material can differ from thewhisker material, and in which none of these different materials,itself, has all of the properties desired but whose combination producesnovel and especially advantageous mechanical configurations.

While it is a general object of this invention to improve theinterlaminar shear properties of many different surface materials bygrowing whiskers directly upon them, it is a particular object of thisinvention to improve carbon fibers, fibrils, threads, mats, or wovencloth in this manner. When these treated fibers are combined with asuitable matrix material, such as a resin, a metal or a ceramic, theimproved interlaminar bond is evidenced by the fact that the resultingcomposite behaves in a manner resembling more nearly an isotropicmaterial.

The Whiskerizing process can produce (1) an improved surface chemistrywhich can be selected to be more compatible with the matrix material,(2) an improved surface geometry providing greater mechanical grip ofthe matrix material on the reinforcements, (3) an integral bond betweenwhisker and supporting surface, and (4) a high degree of reinforcementof the interstitial plastic by whiskers which have been grown in thenormally-void regions adjacent to and between the fibers.

These improvements have been evaluated by testing unidirectionalreinforced bar specimens by the horizontal short-beam shear method.interlaminar shear strengths of at least 11,000 p.s.i. were measured oncomposites which were made of Royal Aircraft Establishments (England) 54million modulus graphite fibers and epoxy resin, which material withoutwhisker growth exhibited a shear value of only about 2500 p.s.i. Shearstrengths of at least 12,000 p.s.i. were achieved on composites madewith Thornel 25 (modulus of 25 million p.s.i.) which showed shear valuesof only about 4,000 p.s.i. before whisker growth. The shear strengthsare, in fact, greater than the strengths quoted because the specimensdid not fail in shear but in bending. The failure was similar to thatexperienced by a homogeneous rather than an anisotropic material.

The process of growing whiskers upon these graphite fibers produced, inthe early test runs, a substantial reduction in the tensile strength ofthe fibers after whisker treatment, some 30% or more. Moreover, therewas a considerable weight loss of the fibers which accompanied the lossin strength and appeared related to it. It is therefore an object ofthis invention to provide an improved Whiskerizing process in which suchweight-loss and loss in tensile strength is minimized by (1) providingcarbon donors inside the treatment oven to reduce the amount of carbonconverted directly from the fibers themselves, and

(2) rapidly cooling the treated fibers as they leave the oven to reducethe tendency thereof to oxidize.

Another object of the invention is to exploit the carbon donortendencies of fibers to make an alternative form of whisker-grown fiberhaving declivities in the fiber surfaces, thereby to roughen it,although at the expense of some loss of tensile strength.

Other objects and advantages of the invention will become apparentduring the following description of specific working examples andvarious apparatus and product configurations shown in the accompanyingdrawings, wherein:

FIG. 1 is a view partly in cross-section of apparatus for carrying outthe present process;

FIG. 2 is a partial view taken along line 2-2 of FIG. 1;

FIG. 3 is an enlarged view of a whisker grown yarn;

FIG. 4 is an enlarged view of a densely whisker grown yarn, resemblingwool;

FIG. 5 is an enlarged view of a pile of whisker grown discreteparticles;

FIG. 6 is an enlarged view of a stack of overlaid fibers of yarn,whisker grown to interweave the stack into a unified mat or rod;

FIG. 7 is an enlarged view of woven fabric whisker grown subsequent toweaving;

FIG. 8 is a view of a second form of apparatus suitable for Whiskerizinga continuous yarn or tape of material being passed through it;

FIG. 9 is a partial sectional view taken along the line 99 of FIG. 8;

FIG. 10 is a view partially in section ofa third form of apparatus forcarrying out the present process;

FIG. 11 is a view partially in section of a fourth form of apparatus forcarrying out the present process; and

FIG. 12 is a view partially in Section of a fifth form of apparatus forcarrying out the present process.

Referring now to the drawings, FIGS. 1 and 2 show typical apparatus forcarrying out the present process by growing whiskers upon various fibermaterials, especially those of graphitic form. The apparatus includes anoven 1, for instance a 20 kw. molybdenum-wound furnace, for establishingand maintaining the necessary high temperatures, generally in the rangeof 2000 to 3000 F. The oven has an inlet duct 2 and an outlet duct 3 sothat its interior can be continuously flushed with hydrogen gas movingto the right at a velocity of about one foot per second in the oven andsupplied by a storage bottle 4. The hydrogen within the oven ismaintained about at atmospheric pressure and has a controlled amount ofwater vapor entrained in it, as will be hereinafter discussed. The oven1 is provided with suitable access doors (not shown).

Within the oven 1 is located a solid carbon or graphite sill 10 whichsupports other apparatus and insulates it against destructive thermalshock when the apparatus is first slid into the hot oven, for instancefrom ambient room temperature. The carbon sill 10 also supports twosolid carbon spacers 11 and 12 upon which a ceramic boat 13 rests. Theboat is a commercially purchased item composed of about alumina, about12% silicon dioxide and about 3% other oxides, and this composition issignificant in some of the examples discussed below. The boat 13contains aluminum shot, 3-5 mesh, and these shot particles melt andprovide a puddle of liquid aluminum 18 in the boat. The boat 13 isclosed by a ceramic cover 14, for instance Johns-Manville 3000 ceramicbrick which is about 60% alumina, 30% silicon dioxide, and 10% otherclays.

The openings between the sides of the boat 13 and the carbon sill 10 areclosed by carbon strips 15 and 16 which, unlike the solid carbonsupports 10, 11 and 12, are quite porous. These strips serve to excludeoxygen from the hollow zone Z between the bottom of the boat 13 and thetop of the sill 10 so as to prevent oxidation of the materials withinthe zone Z when the apparatus is removed hot from the oven. When thisapparatus is being used to treat carbon fibers, the porous strips 15 and16 also serve as carbon donors for the purpose of reducing the amount ofcarbon which the present reactions convert directly from the fibersbeing treated, such carbon conversion representing weight loss of thefibers themselves and corresponding loss in fiber tensile strength,sometimes as much as 30%.

The material to be whisker grown is placed in the zone Z in some cases,or in the area A within the boat between the molten aluminum 18 andcover 14 in other cases, depending upon the chemical composition of thewhiskers sought to be grown. The above apparatus produceswhisker-growing atmospheres in the above-mentioned zone Z and area A,these atmospheres being different and mutually competitive as will bediscussed in connection with the various examples described below.

Since the whisker materials themselves do not vaporize, it is necessaryto use gas transfer mechanisms for growing whiskers. The growing ofsilicon carbide whiskers, for example, requires the presence in theright proportions of silicon monoxide and carbon monoxide, these vaporsbeing obtained by the following reactions involving: the molten aluminumcontained within the boat, the silica and alumina which is part of thecomposition of the boat 13 and cover 14, the carbon of the strips 15 and16, and the hydrogen and water vapor present in the flushing gas:

When these reactions take place in close proximity to carbon fibers,beta silicon carbide whiskers grow out normal to the fiber surfaces.These whiskers are monocrystalline in nature and their rate of growthand final shapes can be strongly influenced by controlling the vaporconcentrations, i.e., the degrees of supersaturation thereof. If theconcentration is low but within the whisker growing range, the siliconcarbide is formed in the shape of long needle-like whiskers, forinstance as shown in FIG. 3, where a yarn 20 of carbon fibers haswhiskers 22 grown out diameters, or more. The individual carbon fibrils21 forming this yarn bundle 20 are about 10 microns in diameter, and thewhiskers 22 are about 0.5 to 1 micron, the former being polycrystalline,and the latter being .monocrystalline and exceptionally perfect incrystal structure.

As the concentration of vapors is increased, whisker density greatlyincreases, for instance to produce much finer silicon carbide whiskersas small as .01 micron in diameter and resembling a dense wool as shownin FIG. 4. Here the yarn 20 of carbon fibrils has a growth of wool 24wherein the population density is very high and the whiskers often forkor branch out so that they intertwine to a great extent. When greatlyenlarged, the structure of FIG. 3 resembles a test-tube brush, whereasthe structure of FIG. 4 looks more like a pipe cleaner. 7 Where theconcentration of vapors is further increased well above the wool-growinglevel, the seeding and vapor precipitation becomes so great that siliconcarbide powder is formed rather than whiskers, and therefore. theconcentration has exceeded the whisker growing range for the particularvapors selected.

When there is sufficient other carbon present during Whiskerizing ofcarbon fibers either as solid carbon, CO, or CH the silicon carbidegrows outwardly from the fiber surfaces, but when the carbon availableto the process is low, the silicon monoxide reacts directly with thecarbon of the fiber substrate, and forms sub-micron silicon carbidepowder by direct conversion of carbon taken from the substrate itself,with the result that silicon carbide grows inwardly into the surface ofthe fibril tend- 6 ing to weaken its tensile strength, although it doesform a silicon carbide coating on the fiber, which coating increases theability of the fiber to bond to plastic matrix materials. The resultingloss of weight of the fiber was a major problem during early efforts togrow whiskers upon carbon fibrils without reducing their modulus.

Practical conditions for whisker growth are illustrated by reference toFIGS. 1 and 2 as follows: The ceramic boat 13 used in early experimentswas 18" long, 5" wide and 2" deep. Its own composition was as set forthabove, and its walls were A" thick. The boat was filled with two poundsof pure aluminum shot and was placed upon a dense graphite sill 10 anddense graphite spacers 11 and 12. Within the oven 1, the temperature wasraised to 2600 F. and the oven was flushed with hydrogen gas atapproximately atmospheric pressure and containing 50 parts per millionof moisture. The gas velocity was about one foot per second. After twohours, the sill and boat were withdrawn from the furnace, andexamination thereof showed that directly beneath the boat and in theimmediate vicinity thereof beta silicon carbide crystals were grown uponthe sill, because the concentration of silicon monoxide vapor was highimmediately adjacent to the boat. Most of the growth, however, was inthe form of sub-micron powder which grew as a result of directconversion of the carbon at the immediate surface of the sill wherecarbon atoms were plentiful and the concentration of SiO was high.However, at a distance somewhat more removed from the boat, theconcentration of SiO was lower, and therefore the silicon carbide grewas whiskers instead of powder. Still further away from the sill, the SiOvapor concentration was reduced to the point where growth ceased. Theabove experiment took place in the absence of any fibers in the zone Zand in the absence of porous carbon strips 15 and 16.

Subsequently porous carbon members 15 and 16 were added in the form ofsheets about A" to /2" thick laid upon the carbon sill beneath the boatas shown in FIGS. 1 and 2. The experiment was re-run using the sameparameters: reducing atmosphere, temperature, and time, and in thisinstance a much greater growth of silicon carbide crystals resultedbecause of the greatly increased carbon surface provided by the poroussheets 15 and 16 which then resulted in the production of adequatecarbon monoxide. This time, the growth was in the form of whiskersrather than powder, the growth being especially thick upon the porouscarbon sheets, but also distributed throughout the pores of the sheets.The degree of penetration of whisker growth is diffusion dependent, thegrowth penetrating about A" of 50% porous carbon per hour at 2600 F. Itwas also found that by changing the parameters to increase thetemperature, or by increasing the porosity of the carbon, thepenetration of the whisker growth within the porous carbon sheets wasgreatly accelerated. With the above as background, the followingexamples of whisker growing will be illustrative of the process andresulting products.

EXAMPLE I Using the same parameters as set forth in the precedingseveral paragraphs, powdered carbon was piled within the zone Z betweenthe solid carbon spacers 11 and 12 and the porous strips 15 and 16, andwhiskers were then grown upon the powdered carbon for sufiicient time topermit growth penetrating throughout the carbon pile. The type of growthwhich resulted is shown in FIG. 5. The individual carbon particles 30were Whiskerized so that a dense intergrowth formed a complete whiskernetwork around all of the carbon particles, these whiskers intertwiningto form a novel lightweight porous structure which no longer exhibitedthe general characteristics of a loose powder but had a great deal ofstructural integrity. This resulting product was then transformed into asponge of interlocked beta silicon carbide whiskers by heating theproduct to 1600 for a period of four to six hours in the presence ofoxygen to burn off the unconverted carbon particles and leave only thewhiskers.

EXAMPLE II In this example, the zone Z bounded by the solid carbonspacers 11 and 12 and the porous carbon strips 15 and 16 was occupied bysuitable support means strung with bundles of high modulus graphitefiber in the form of a yarn of the type shown in FIG. 3 composed of alarge number of IO-micron fibrils, and the porous carbon strips 15 and16 were brought into fairly close proximity thereto so as to act ascarbon donors. Several such experiments were then run to produce siliconcarbide whisker growth as shown in FIGS. 3 and 4, depending upon theconcentration of vapors and the length of time the experiment was run.The general experiment was then re-run using mats of fibers or yarnstacked on top of each other as shown in FIG. 6 to form athree-dimensional rod which is ap proximately /2" x /2 x A", weighingabout 20 grams. The particular fibers used were of the type made byUnion Carbide under the trade name Thornel 25 or Thornel 40, or made byHitco under the designation HGM 25, or made by the British Royal AirForce Establishment. The fibers were of carbon in highly crystallizedgraphite form having very smooth external surfaces which do not easilyconvert as carbon donors, and most of the carbon was therefore takenfrom the porous strips 15 and 16, which were brought close to the rodbeing treated. The stack of fibers forming the rod is generally referredto by the reference numeral 35 in FIG. 6 and comprises individual carbonfibers and bundles 37 with a dense growth of whisker 38 extendingbetween them. The time required to grow the whiskers throughout the rod35 depended somewhat upon the density with which the fibers were packedtogether when forming the rod but good whisker growth occurred withinone hour. The whisker growth was about 1 to in weight. AfterWhiskerizing, the rod of fibers which were initially loosely associatedhad grown together to form an integral bundle which handled very much asthough it was bound together. This type of rod when impregnated with asuitable matrix material provides a very high strength structure.

As possible modifications of the present example, the fibers can be laidtogether in random directions to form a kind of felt, or they can becrossed in alternate layers to form parallel unidirectional sheetsalternating in the X direction and in the Y direction as the sheet isbeing stacked. The X and Y fibers may or may not be of identicalmaterials, but in any event, after Whiskerizing, the resulting compositeno longer resembles loose layers of fibers, but is so well intergrownthat it becomes a three dimensional bi-fiber composite of whiskers whichnot only link the various fibers together, but also grow to fill thevoid spaces between the various fibers and bundles, the voids 36, forinstance, being larger than the voids 39, and all voids beingsecondarily reinforced by the growth of whiskers which, when thestructure is impregnated by a plastic resin, will greatly reinforce theresin occupying the voids. In this way the resulting composite isstrengthened in an additional manner which is quite beyond the degree ofstrengthening which could be provided by mere roughening of the originalfibers and bundles to increase their ability to bond.

EXAMPLE III In this example, the zone Z was occupied by one or morelayers of woven cloth or tape sometimes overlaid in groups including asmany as layers. For this purpose, multiple layers of woven graphitecloth, Hitco type C-cc-IA were placed under the ceramic boat, and thiscloth was then Whiskerized under the same conditions as set forth abovewith the result that beta silicon carbide whiskers were grown extendingin all directions from the carbon fibril surfaces. These whiskers grewto fill in all the interstitial voids within the weave as well asbetween the carbon cloth layers, and these whiskers taken with the clothyarns formed an intergrown network of two different characters, namelythe carbon fibers themselves, and the silicon carbide whiskers whichgrew so long and in such density that a new type of reinforcing productwas formed, namely a three dimensional bi-fiber fabric.

EXAMPLE IV In this example, alumina fibers were placed in the area A,within the boat and between the molten aluminum 18 and the ceramic cover14, and in this area the same operational parameters resulted in adifierent process used to grow sapphire whiskers on the aluminasurfaces. Within the aluminum boat the presence of water vapor and thepresence of silicon monoxide provides a reaction which forms sapphirecrystals A1 0 growing upon the alumina fiber surfaces. The conditionsfor the growth of sapphire crystals are repugnant to the growth ofsilicon carbide crystals and vice versa. The silicon carbide crystalsare grown in the presence of plentiful carbon donors in the form ofporous strips 15 and 16 which contribute to the formation of carbonmonoxide. However, although plentiful in zone Z, carbon monoxide isvirtually absent from area A, FIG. 1, and therefore silicon carbide isnot formed. Rather, sapphire crystals are formed on the fiber surfacesinside area A. Sapphire crystals can also be grown on zirconia, titaniumoxide, molybdenum, etc., these materials likewise being placed withinthe boat just above the molten aluminum in the absence of carbon donors.A further listing of compatible whisker materials and substratematerials will appear hereinafter in the specification.

EXAMPLE V This example is given with reference to FIGS. 8 and 9, andshows the continuous processing of fibers fed through an oven 41 havingsuitable heating means (not shown) and having an inlet duct 42 and anexhaust duct 43 through which a hydrogen gas reducing atmosphere can becirculated. These members are connected to horizontal passageways 42aand 43a which are suitably shaped to receive a continuous run of fibersP which passes through the oven between driven spools 38 and 39. Insidethe oven, a ceramic boat 44 is provided with a cover 45 and sits upon asill 46, all of which parts are the same as those shown in FIGS. 1 and2. The boat 44 is actually supported on two porous carbon bars 47 and 48which not only support the boat 44 but also serve as carbon donors inthe treatment of the fibers F as they pass through the oven. The boatcontains molten aluminum 49, and the entire system is raised to thetemperature of 2600 F. by suitable means (not shown). The fibers F maycomprise a singel bundle, a plurality of parallel bundles, or a wovencloth or tape for purposes of the present illustration, there being nobasic difference in the process. The passageways 42a and 43a can beprovided with air locks at their extreme outer ends, although the airlocks are really not necessary since the hydrogen gas will escapeoutwardly at these points and discourage the entry of atmospheric gasesinto the oven. The fibers F pass between the bottom of the boat 44 andthe top of the sill 46 and between the porous carbon bars 47 and 48,which in the actual experiment were 1" x 1" x 20". The carbon yarn wassufiiciently Whiskerized for the purpose of adequately increasing itsinterlaminar shear capabilities after 10 minutes at 2600 F. inside theoven; while at 2800 F. less than two minutes was required. No doubtimproved parameters can be used to reduce the transit time even further,for instance, by adding 00 or CH to the gas stream to increase thecarbon donors. More'- over, a more eflicient oven structure can bedevised to process multiple strands or tapes simultaneously whilepassing them through paths of greater linear length. When multipleadjacent yearns were passed through the oven together in adjacenttransverse mutual contact, they emerged as a unified ribbon because ofintertwining of the grown whiskers. When woven tapes were passed throughThe apparatus shown in FIGS. 1 and 2 can be utilized to grow molybdenumsilicide whiskers on molybdenum Wire by supporting the latter adjacentto a carbon sill and placing the boat 13 directly over the molybdenumwire in the zone Z. The oven is heated to 2800 F. and flushed with an Hgas stream as in the previous examples, the process being allowed .tocontinue, for example for an hour. These reactions include:

Al SiOe A10 SiO H2 2Si0 Mo MOSlz 2H20 The resulting molybdenumdisilicide whiskers are from .5 to 10 microns in diameter and up to afew millimeters in length.

EXAMPLE VII This example refers to FIG. 10 and shows the growing ofboron carbide whiskers on carbon filaments within a graphite-tubefurnace 50 heated hotter near its left end by a resistance wire coil 51.Argon gas under a vacuum of 50 microns is flushed through the furnace inthe direction of the arrow 52., and flows over a graphite boat 53containing powdered boron carbide B B. The gas then flows over thecarbon filaments 54 being treated, this example showing an open meshcylinder of carbon filaments 54 which, when fWhiskerizedf will resembleFIG. 7. These whiskers are grown by the pure vapor method wherein B Cpowder is heated at reduced pressure to about 3 500 F., and its vapor isthen condensed downstream at a lower temperature of about 3150 F. uponthe carbon mesh 54. The process takes from about 30 minutes to a fewhours depending upon the desired whisker length.

'EXAMPLE VIII This example demonstrates the growth of silicon nitride SiN whiskers on silicon carbide or on graphite filaments, using apparatusillustrated in FIG. 11. A hydrogen atmosphere furnace 60 is used to heatporous high-silica brick 61 in the presence of other gases introducedinto the furnace as shown schematically by the arrow 62. These othergases include hydrogen H 100 parts; ammonia NH 30 parts; and methane CHone part. The process takes place preferably at a slowly risingtemperature ranging from 2550 F. to 2650 F. Compatible fibers 63, suchas carbon or silicon carbide, when placed in the whisker growth region Rbetween the closely spaced bricks 61 will be Whiskerized according tothe following reactions over a period of one to six hours.

Actually nascent nitrogen may not be present but may form a complex withthe methane which acts as a promoter. In this event a set of reactionsmay take place as follows:

The small percentage or methane gas is added as a promoter of thedecomposition of the ammonia gas to release the nitrogen which in turnreacts with the silicon monoxide to produce the Si 'N whiskers. Thefurnace heating means is not shown in the present illustration.

EXAMPLE IX This example shows the growing of metal whiskers upon gridsor filaments which can be either metallic or ceramic. For instancecopper whiskers can be brown on stainless steel screening by a processinvolving the reduction of a halide salt using the apparatus shown inFIG. 12.

This reaction takes place in a quartz tube 70, which is flushed with dryhydrogen gas as represented by the arrow 71, which gas flows over a boat72 of heated copper iodide crystals to entrain vapor and pass it overthe wires of the stainless steel screening 73. The vicinity of the boat72 is heated, for instance by a resistance wire 74, to about 1400 F.,but the deposition of copper takes place in a cooler Whiskerizingsection S of the apparatus, at about 1000 F.

From the above examples, it can be seen that Whiskerizing can beaccomplished either in systems where some of the elements are common, ornone of the elements arev common, to both the whiskers and the fibers.Examples of common-element systems include: carbon or graphite whiskerson carbon or graphite; silicon carbide whiskers on silicon (or onsilicon dioxide, or on silicon carbide); molybdenum disilicide whiskerson molybdenum; tungsten disilicide whiskers on tungsten; boron carbidewhiskers on carbon; boron whiskers on boron carbide or on boron nitride.

The other type of system includes entirely different elements in thewhiskers and in the substrates. Examples thereof include: siliconcarbide whiskers on boron; sapphire whiskers on zirconium oxide; siliconnitride whiskers on carbon; copper whiskers on tungsten; and ironwhiskers on molybdenum. The number of compatible combinations is almostlimitless. It should be noted that the present reactions are not to belimited to an H atmosphere. It is only necessary that the atmosphere benon-oxidizing. Moreover, care must be used to avoid incompatiblecombinations such as carbon whiskers on iron, carbon being soluble iniron; or sapphire whiskers on carbon, the necessary atmosphere oxidizingthe carbon too quickly.

The present invention is not to be limited to the illustrative examplesand drawings, for obviously the scope of the claims is much greater.

We claim:

1. The method of treating the surface of a filament material by growingcrystalline whiskers thereon .to increase the bonding capability thereofwhen laminated to another substance, including the steps of:

(a) heating the surface to be treated to an elevated temperature belowthe vaporization temperature of said whiskers in the presence of anon-oxidizing atmosphere;

(b) generating in said atmosphere whisker vapors or precursore thereof:

\(C) applying said vapors in supersaturated concentration in thevicinity of said surface to deposit seed crystals and iniate whiskergrowth; and

(d) continuing said heating and said vapor application for a period oftime sufiicient at least to substantially roughen said surface.

2. The method as set forth in claim 1, wherein the degree ofsupersaturation ofthe vapors near said surface is maintained near theminimum concentration required for whisker growing, whereby a relativelysparse population of whiskers of relatively large cross-section for thevapors selected is grown.

3. The method as set forth in claim 1, wherein the degree ofsupersaturation of the vapors near said surface is maintained near themaximum concentration required for whisker growing, whereby a relativelydense population of whiskers of relatively small cross-section for thevapors selected is grown.

4. The method as set forth in claim 1, including the step ofcontinuously feeding said material through the vicinity of said vaporconcentration at a rate selected to leave each increment of the materialin said vicinity long enough to substantially change its surface.

5. The method as set forth in claim 1 for treating the treating thesurfaces of carbonaceous materials to grow whiskers thereon consistingof other compounds of carbon, including the step of grouping thecarbonaceous material to be treated in close proximity with substantialother carbon-donor materials while applying said vapors, whereby thedonors serve' to reduce the amount of carbon converted from thematerials being treated while forming said other compounds.

6. The method as set forth in claim 1 for surface-treating carbonaceousmaterial to grow whiskers thereon and change their surface chemistry,including the steps of placing the carbonaceous material in anenvironment starved for carbon donors; and generating in said atmosphereat about 2600-2800 F. silico monoxide vapor, whereby silicon carbide isformed on the carbonaceous surfaces by conversion of carbon takendirectly from the latter surfaces.

7. The method of forming a reinforcement structure of whisker-grownmaterial, including the steps of:

(at) bringing together a plurality of fibers of said material to form abundle having interstitial voids between the fibers; and

(b) subsequently treating the fibers according to the method as setforth in claim 1 for a period of time long enough to grow the whiskerswell out from the fiber surfaces into the voids.

8. The method as set forth in claim 7, wherein said period of time islong enough to grow whiskers into mutually intertwining relationshipacross most of said interstitial voids.

9. The method as set forth in claim 7, including the steps of stackingbundles of fibers shaped in three dimen- I2 sions to providea-reinforcement structure; and subsequently treating thestructure togrow-said Whiskers.

10. The method asset forth in claim 7, including the steps of stackingbundles of said fibers in alternate layers whose fiber directions aremutually crossed, and subsequently treating the stacked bundles to growsaid whiskers.

11. The method of forming a reinforcement structure of whisker-grownmaterial, including the steps of:

1 (a) forming fabric of yarn made of said material, the fabric havinginterstitial voids in and around said yarn; and I (b) subsequentlytreatingthe fabric material according to the method as set forth inclaim 1 for a period of time long enough'to grow whiskers well out fromthe material into said voids.

12. The method as set forth in claim 11, wherein said period of time islong enough to grow whiskers into mutually intertwining relationshipacross most of said interstitial voids.

13. The method asset forth in claim 11, including the steps of stackinglayers of said fabric to form a reinforcement structure shaped in threedimensions, and subsequently treating the shapedstructure to grow saidwhiskers.

14. The method of forming a three dimenional reinforcement structure,including the steps of: v

(a) forming particles of said material into a desired overall shape; and

(b) subsequently treating the particles according to the method as setforth in claim 1 for a period of time long enough to grow whiskers outfrom the particle surfaces and mutually intertwined sufiiciently tomaintain said desired shape.

References Cited UNITED STATES PATENTS 3,246,950 4/ 1966 Gruber 232083,275,415 9/ 1966 Chang et al 23'-208 3,306,705 2/ 1967 Leineweber et a123208 3,3653 30 1/1968 Hough '117106X 3,369,920 2/1968 Bourdeau et al117106X 3,459,504 8/ 1969 Bracken et al 23208 ROBERT F. BURNETT, PrimaryExaminer R. L. MAY, Assistant Examiner US. Cl. X.R.

